Electrode for electrochemical device, electrochemical device, and method of producing electrode for electrochemical device

ABSTRACT

Provided is an electrode for an electrochemical device that has excellent peel strength and can ensure a high level of safety of an electrochemical device. The electrode for an electrochemical device includes a current collector and an electrode mixed material layer on the current collector. The electrode mixed material layer contains an electrode active material, a binder, and a foaming agent. The binder is a polymer including a diene monomer unit and/or nitrile group-containing monomer unit, and in which the total proportion constituted by the diene monomer unit and nitrile group-containing monomer unit is 10 mass % to 80 mass %. Volume resistivity R A  of a laminate of the electrode mixed material layer and current collector at 25° C. is 0.1 Ω·cm to 200 Ω·cm, and a ratio of volume resistivity R B  of the laminate at 350° C. relative to volume resistivity R A  of the laminate at 25° C. is 10 or more.

TECHNICAL FIELD

The present disclosure relates to an electrode for an electrochemicaldevice, an electrochemical device, and a method of producing anelectrode for an electrochemical device.

BACKGROUND

Electrochemical devices such as secondary batteries, primary batteries,and capacitors have been used in a wide variety of applications inrecent years. An electrochemical device includes, for example, aplurality of electrodes obtained by forming an electrode mixed materiallayer on a current collector, and a separator that isolates theseelectrodes from one another.

There are cases in which thermal runaway of an electrochemical devicemay occur due to a short circuit between electrodes, overcharging, orthe like. For this reason, attempts have been made to ensure the safetyof electrochemical devices even when a short circuit between electrodesor overcharging occurs.

For example, Patent Literature (PTL) 1 discloses that with respect to anon-aqueous secondary battery including electrodes (positive electrodeand negative electrode), a separator, a non-aqueous electrolytesolution, a case housing these components, and a safety mechanism thatis sensitive to and actuated by a rise in internal pressure of thebattery, excellent safety during overcharging is ensured through theinclusion of an organic chemical foaming agent inside the case. Morespecifically, PTL 1 discloses that as a result of inclusion of theorganic chemical foaming agent, gas evolves from the organic chemicalfoaming agent in an initial stage of overcharging, leading to a rise inbattery internal pressure and reliable actuation of the safetymechanism.

CITATION LIST Patent Literature

PTL 1: JP 2006-73308 A

SUMMARY Technical Problem

However, it has not been possible to ensure sufficient safety of anelectrochemical device with the conventional method described in PTL 1.For example, in a situation in which a short circuit occurs between apositive electrode and a negative electrode in a non-aqueous secondarybattery described as an electrochemical device in PTL 1, it is notpossible to respond to a rapid rise in temperature caused by the shortcircuit, and ignition and cell rupture caused by thermal runaway cannotbe sufficiently inhibited.

Moreover, an electrode of an electrochemical device is required to havestrong close adherence between the electrode mixed material layer andcurrent collector thereof (i.e., have excellent peel strength). However,it has not been possible to ensure excellent peel strength in theelectrode described in PTL 1.

In other words, there is room for improvement of the technique describedin PTL 1 in terms of ensuring a high level of safety of anelectrochemical device and increasing the peel strength of an electrode.

Accordingly, an objective of the present disclosure is to provide ameans for advantageously solving the problems set forth above.

Solution to Problem

The inventors conducted diligent investigation with the aim of solvingthe problems set forth above. The inventors discovered that the peelstrength of an electrode can be increased and a high level of safety ofan electrochemical device including the electrode can be ensured when anelectrode active material, a specific binder, and a foaming agent arecontained in an electrode mixed material layer, when the volumeresistivity at 25° C. of a laminate in which the electrode mixedmaterial layer and a current collector are stacked (hereinafter, alsoreferred to as an “electrode laminate”) is within a specific range, andwhen a ratio of the volume resistivity of the electrode laminate at 350°C. relative to the volume resistivity of the electrode laminate at 25°C. is not less than a specific value. In this manner, the inventorscompleted the present disclosure.

Specifically, the present disclosure aims to advantageously solve theproblems set forth above by disclosing an electrode for anelectrochemical device comprising a current collector and an electrodemixed material layer on the current collector, wherein the electrodemixed material layer contains an electrode active material, a binder,and a foaming agent, the binder is a polymer including either or both ofa diene monomer unit and a nitrile group-containing monomer unit, and inwhich a proportion constituted by the diene monomer unit and aproportion constituted by the nitrile group-containing monomer unit are,in total, not less than 10 mass % and not more than 80 mass %, volumeresistivity R_(A) of a laminate of the electrode mixed material layerand the current collector at 25° C. is not less than 0.1 Ω·cm and notmore than 200 Ω·cm, and a ratio of volume resistivity R_(B) of thelaminate at 350° C. relative to the volume resistivity R_(A) of thelaminate at 25° C. is 10 or more. An electrode for which the electrodemixed material layer contains an electrode active material, a binderformed by the polymer set forth above, and a foaming agent, and forwhich the electrode laminate has a volume resistivity R_(A) (25° C.)that is within the specific range set forth above while also having aratio of volume resistivity R_(B) (350° C.) relative to the volumeresistivity R_(A) (25° C.) that is not less than the specific value setforth above in this manner has excellent peel strength and can providean electrochemical device with a high level of safety.

The phrase “includes a monomer unit” as used in the present disclosuremeans that “a polymer obtained with the monomer includes a repeatingunit derived from the monomer”.

Moreover, the “volume resistivity” referred to in the present disclosurecan be measured by a method described in the EXAMPLES section of thepresent specification.

In the presently disclosed electrode for an electrochemical device, thefoaming agent preferably has a foaming temperature of not lower than100° C. and not higher than 350° C. When the foaming temperature of thefoaming agent is within the range set forth above, rate characteristicsand high-temperature storage characteristics of an electrochemicaldevice can be enhanced, and a high level of safety of theelectrochemical device can sufficiently be ensured.

The “foaming temperature” referred to in the present disclosure can bemeasured by a method described in the EXAMPLES section of the presentspecification.

In the presently disclosed electrode for an electrochemical device, theelectrode mixed material layer preferably contains not less than 0.01parts by mass and not more than 10 parts by mass of the foaming agentper 100 parts by mass of the electrode active material. When the contentof the foaming agent in the electrode mixed material layer is within therange set forth above, the peel strength of the electrode can be furtherimproved. It is also possible to enhance rate characteristics andsufficiently ensure a high level of safety of an electrochemical device.

The “content of a foaming agent in an electrode mixed material layer”referred to in the present disclosure can be calculated by measuringnitrogen content by the combustion method (modified Dumas method) or thelike in a case in which the foaming agent is an organonitrogen foamingagent such as described further below, for example, and can becalculated by measuring the released amount of carbon dioxide gas usinga temperature-programmed desorption gas analyzer or the like in a casein which the foaming agent is an inorganic foaming agent such asdescribed further below, for example. More specifically, the “content ofa foaming agent in an electrode mixed material layer” referred to in thepresent disclosure can be measured by a method described in the EXAMPLESsection of the present specification.

In the presently disclosed electrode for an electrochemical device, thefoaming agent is preferably an organonitrogen foaming agent. When anorganonitrogen foaming agent is used as the foaming agent, ratecharacteristics of an electrochemical device can be enhanced, and a highlevel of safety of the electrochemical device can sufficiently beensured.

In the presently disclosed electrode for an electrochemical device, thepolymer preferably has a glass-transition temperature of not lower than−30° C. and not higher than 100° C. When the glass-transitiontemperature of the polymer serving as the binder is within the range setforth above, the peel strength of the electrode can be further improved.It is also possible to enhance high-temperature storage characteristicsand sufficiently ensure a high level of safety of an electrochemicaldevice.

The “glass-transition temperature” referred to in the present disclosurecan be measured by a method described in the EXAMPLES section of thepresent specification.

In a case in which the electrode active material of the presentlydisclosed electrode for an electrochemical device is a positiveelectrode active material, the volume resistivity R_(A) of the laminateat 25° C. is preferably not less than 10 Ω·cm and not more than 180Ω·cm. When the volume resistivity R_(A) (25° C.) is within the range setforth above in a case in which the electrode for an electrochemicaldevice contains a positive electrode active material as the electrodeactive material (i.e., in a case in which the electrode for anelectrochemical device is a positive electrode for an electrochemicaldevice), rate characteristics of an electrochemical device including thepositive electrode can be improved.

In a case in which the electrode active material of the presentlydisclosed electrode for an electrochemical device is a negativeelectrode active material, the volume resistivity R_(A) of the laminateat 25° C. is preferably not less than 0.2 Ω·cm and not more than 50Ω·cm. When the volume resistivity R_(A) (25° C.) is within the range setforth above in a case in which the electrode for an electrochemicaldevice contains a negative electrode active material as the electrodeactive material (i.e., in a case in which the electrode for anelectrochemical device is a negative electrode for an electrochemicaldevice), rate characteristics of an electrochemical device including thenegative electrode can be improved.

Moreover, the present disclosure aims to advantageously solve theproblems set forth above by disclosing an electrochemical devicecomprising any one of the electrodes for an electrochemical device setforth above. When an electrochemical device includes any one of theelectrodes set forth above, thermal runaway of the electrochemicaldevice is inhibited even when a short circuit occurs between electrodes,for example, and a high level of safety of the electrochemical device isensured.

Furthermore, the present disclosure aims to advantageously solve theproblems set forth above by disclosing a method of producing any one ofthe electrodes for an electrochemical device set forth above,comprising: applying a slurry composition for an electrode mixedmaterial layer containing the electrode active material, the binder, thefoaming agent, and a solvent onto the current collector; and drying theslurry composition for an electrode mixed material layer applied on thecurrent collector at a temperature of not lower than 50° C. and nothigher than 130° C. to form an electrode mixed material layer. Anelectrode that is obtained as set forth above has excellent peelstrength and can provide an electrochemical device with a high level ofsafety.

Advantageous Effect

According to the present disclosure, it is possible to provide anelectrode for an electrochemical device that has excellent peel strengthand can ensure a high level of safety of an electrochemical device, andalso to provide a method of producing this electrode for anelectrochemical device.

Moreover, according to the present disclosure, it is possible to providean electrochemical device in which a high level of safety is ensured.

DETAILED DESCRIPTION

The following provides a detailed description of embodiments of thepresent disclosure.

The presently disclosed electrode for an electrochemical device can beused as an electrode of an electrochemical device such as a secondarybattery, a primary battery, or a capacitor. Moreover, the presentlydisclosed electrode for an electrochemical device can be produced, forexample, using the presently disclosed method of producing an electrodefor an electrochemical device. Furthermore, the presently disclosedelectrochemical device includes the presently disclosed electrode for anelectrochemical device.

(Electrode for Electrochemical Device)

The presently disclosed electrode for an electrochemical device includesan electrode laminate in which an electrode mixed material layer isstacked at one side or both sides of a current collector. Note thatlayers other than the electrode mixed material layer and the currentcollector (hereinafter, referred to as “other layers”) may be includedat the surface of the electrode (particularly the surface at theelectrode mixed material layer-side).

At least an electrode active material, a binder, and a foaming agent arecontained in the electrode mixed material layer of the presentlydisclosed electrode. The binder in the electrode mixed material layer isa polymer including either or both of a diene monomer unit and a nitrilegroup-containing monomer unit, and in which the proportion constitutedby the diene monomer unit and the proportion constituted by the nitrilegroup-containing monomer unit are, in total, not less than 10 mass % andnot more than 80 mass %. Moreover, in the presently disclosed electrode,volume resistivity R_(A) (25° C.) of the electrode laminate is not lessthan 0.1 Ω·cm and not more than 200 Ω·cm, and a ratio of volumeresistivity R_(B) (350° C.) of the electrode laminate relative to volumeresistivity R_(A) (25° C.) of the electrode laminate is 10 or more.

The presently disclosed electrode has excellent peel strength and canprovide an electrochemical device with a high level of safety as aresult of containing the foaming agent and specific binder set forthabove in the electrode mixed material layer, and as a result of thevolume resistivity R_(A) (25° C.) of the electrode laminate being withina range of 0.1 Ω·cm to 200 Ω·cm and the value of volume resistivityR_(B) (350° C.)/volume resistivity R_(A) (25° C.) being 10 or more. Thereason that the presently disclosed electrode for an electrochemicaldevice has excellent peel strength and can be used to ensure a highlevel of safety of an electrochemical device as described above ispresumed to be as follows.

Firstly, the polymer that is contained in the electrode mixed materiallayer as the binder has excellent strength as a result of includingeither or both of a diene monomer unit and a nitrile group-containingmonomer unit in proportions such that the total amount thereof is withina specific range. The polymer having excellent strength in this mannerdisplays high binding capacity. In addition, this polymer can stronglyadhere components (electrode active material, etc.) in the electrodemixed material layer to the current collector through interactions withthe foaming agent. Therefore, the inclusion of both the polymer havingexcellent binding capacity and the foaming agent in the electrode mixedmaterial layer can increase the peel strength of the electrode. On theother hand, the foaming agent contained in the electrode mixed materiallayer contributes to improving the peel strength of the electrode asmentioned above and is also thought to contribute to safety of anelectrochemical device. More specifically, when thermal runaway of anelectrochemical device occurs and the temperature inside the devicerises, the foaming agent in the electrode mixed material layer foams andreleases incombustible gas. The released incombustible gas can preventthe spreading of fire by diluting combustible gas evolved throughdecomposition of electrolyte solution or the like due to the hightemperature. Moreover, when the foaming agent foams to releaseincombustible gas, the strong adhesion through interactions between thefoaming agent and the polymer described above is lost. This may causedestruction of electrode structure (for example, detachment of theelectrode active material from the current collector) and cut off aconduction path. As a result, it is possible to inhibit generation ofJoule heat and suppress a further rise in temperature. In addition,since the volume resistivity R_(A) (25° C.) of the electrode laminate inthe presently disclosed electrode is within a range of 0.1 Ω·cm to 200Ω·cm, conductivity at around normal temperature is ensured, and devicecharacteristics of an electrochemical device are ensured. Furthermore,the value of volume resistivity R_(B) (350° C.)/volume resistivity R_(A)(25° C.) for the presently disclosed electrode is 10 or more. In otherwords, the presently disclosed electrode can, through the contributionof the foaming agent described above, rapidly increase the volumeresistivity thereof and inhibit passing of current at high temperature,and thus can sufficiently ensure safety of an electrochemical deviceeven when the inside of the electrochemical device reaches a hightemperature.

<Electrode Mixed Material Layer>

The electrode mixed material layer contains an electrode activematerial, a binder, and a foaming agent. Also note that the electrodemixed material layer may contain components other than the electrodeactive material, the binder, and the foaming agent (hereinafter,referred to as “other components”).

<<Electrode Active Material>>

Known electrode active materials that are used in electrochemicaldevices may be used as the electrode active material without anyspecific limitations. Specifically, examples of electrode activematerials that can be used in an electrode mixed material layer of alithium ion secondary battery, which is one example of anelectrochemical device, include, but are not specifically limited to,the electrode active materials described below.

[Positive Electrode Active Material]

Examples of positive electrode active materials that can be compoundedin a positive electrode mixed material layer of a positive electrode ina lithium ion secondary battery include transition metal-containingcompounds such as transition metal oxides, transition metal sulfides,and complex metal oxides of lithium and transition metals. Thetransition metal may, for example, be Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo,or the like.

Specific examples of positive electrode active materials include, butare not specifically limited to, lithium-containing cobalt oxide(LiCoO₂), lithium manganate (LiMn₂O₄), lithium-containing nickel oxide(LiNiO₂), lithium-containing complex oxide of Co-Ni-Mn,lithium-containing complex oxide of Ni-Mn-Al, lithium-containing complexoxide of Ni-Co-Al, olivine-type lithium iron phosphate (LiFePO₄),olivine-type lithium manganese phosphate (LiMnPO₄), lithium-rich spinelcompounds represented by Li_(1−x)Mn_(2−x)O₄ (0<x<2),Li[Ni_(0.17)Li_(0.2)Co_(0.07)Mn_(0.56)]O₂, and LiNi_(0.5)Mn_(1.5)O₄.

Of these positive electrode active materials, lithium-containing complexoxide of Co-Ni-Mn and lithium-containing complex oxide of Ni-Co-Al arepreferable from a viewpoint of improving safety of a lithium ionsecondary battery.

One of these positive electrode active materials may be usedindividually, or two or more of these positive electrode activematerials may be used in combination.

[Negative Electrode Active Material]

Examples of negative electrode active materials that can be compoundedin a negative electrode mixed material layer of a negative electrode ina lithium ion secondary battery include carbon-based negative electrodeactive materials, metal-based negative electrode active materials, andnegative electrode active materials containing any combination thereof.Herein, “carbon-based negative electrode active material” refers to anactive material having a main framework of carbon into which lithium canbe inserted (also referred to as “doping”). Specific examples ofcarbon-based negative electrode active materials include carbonaceousmaterials such as coke, mesocarbon microbeads (MCMB), mesophasepitch-based carbon fiber, pyrolytic vapor-grown carbon fiber, pyrolyzedphenolic resin, polyacrylonitrile-based carbon fiber, quasi-isotropiccarbon, pyrolyzed furfuryl alcohol resin (PFA), and hard carbon, andgraphitic materials such as natural graphite and artificial graphite.

A metal-based negative electrode active material is an active materialthat contains metal, the structure of which usually contains an elementthat allows insertion of lithium, and that has a theoretical electriccapacity per unit mass of 500 mAh/g or more when lithium is inserted.Examples of metal-based active materials include lithium metal, simplesubstances of metals that can form lithium alloys (for example, Ag, Al,Ba, Bi, Cu, Ga, Ge, In, Ni, P, Pb, Sb, Si, Sn, Sr, Zn, and Ti), andoxides, sulfides, nitrides, silicides, carbides, and phosphides thereof.Also, an oxide such as lithium titanate can be used.

Of these negative electrode active materials, graphitic materials suchas natural graphite and artificial graphite are preferable.

One of these negative electrode active materials may be usedindividually, or two or more of these negative electrode activematerials may be used in combination.

<<Binder>>

The binder is a component that closely adheres the electrode mixedmaterial layer to the current collector and can inhibit detachment ofcomponents such as the electrode active material and the subsequentlydescribed foaming agent from the electrode mixed material layer.

The electrode mixed material layer of the presently disclosed electrodeis required to contain a specific polymer as the binder. The polymerserving as the binder includes a diene monomer unit and/or a nitrilegroup-containing monomer unit as a repeating unit. Moreover, when theamount of all repeating units included in the polymer (i.e., the totalamount of all monomer units and all structural units) is taken to be 100mass %, the proportion constituted by the diene monomer unit and theproportion constituted by the nitrile group-containing monomer unit are,in total, not less than 10 mass % and not more than 80 mass %. Thepolymer serving as the binder includes at least 20 mass % of repeatingunits other than the diene monomer unit and the nitrile monomer unit(hereinafter, referred to as “other repeating units”).

[Diene Monomer Unit and Nitrile Group-Containing Monomer Unit]

Diene Monomer Unit

Examples of diene monomers that can form the diene monomer unit includealiphatic conjugated diene monomers. Examples of aliphatic conjugateddiene monomers include 1,3-butadiene, isoprene,2,3-dimethyl-1,3-butadiene, and 1,3-pentadiene. Of these aliphaticconjugated diene monomers, 1,3-butadiene is preferable. One dienemonomer may be used individually, or two or more diene monomers may beused in combination.

In a case in which the presently disclosed electrode for anelectrochemical device is a negative electrode for an electrochemicaldevice, it is preferable that the polymer includes a diene monomer unit.The proportion constituted by the diene monomer unit when the amount ofall repeating units included in the polymer in the negative electrodemixed material layer is taken to be 100 mass % is preferably 15 mass %or more, and more preferably 20 mass % or more, and is preferably 70mass % or less, more preferably 50 mass % or less, and even morepreferably 40 mass % or less. The peel strength of the electrode can befurther improved when the proportion constituted by the diene monomerunit in the polymer contained in the negative electrode mixed materiallayer is 15 mass % or more. On the other hand, a high level of safety ofan electrochemical device can sufficiently be ensured as well as furtherimproving the peel strength of the electrode when the proportionconstituted by the diene monomer unit in the polymer contained in thenegative electrode mixed material layer is 70 mass % or less.

Moreover, in a case in which the presently disclosed electrode for anelectrochemical device is a negative electrode for an electrochemicaldevice, the polymer is preferably a polymer (aliphatic conjugateddiene/aromatic vinyl copolymer) that includes both an aliphaticconjugated diene monomer unit as a diene monomer unit and a subsequentlydescribed aromatic vinyl monomer unit, and in which the total proportionconstituted by these two types of monomer units is more than 50 mass %.

Nitrile Group-Containing Monomer Unit

Examples of nitrile group-containing monomers that can form the nitrilegroup-containing monomer unit include α,β-ethylenically unsaturatednitrile monomers. Specifically, any α,β-ethylenically unsaturatedcompound that has a nitrile group can be used as an α,β-ethylenicallyunsaturated nitrile monomer without any specific limitations. Examplesinclude acrylonitrile; α-halogenoacrylonitriles such asα-chloroacrylonitrile and α-bromoacrylonitrile; andα-alkylacrylonitriles such as methacrylonitrile andα-ethylacrylonitrile. Of these α, β-ethylenically unsaturated nitrilemonomers, acrylonitrile is preferable. One nitrile group-containingmonomer may be used individually, or two or more nitrilegroup-containing monomers may be used in combination.

In a case in which the presently disclosed electrode for anelectrochemical device is a positive electrode for an electrochemicaldevice, it is preferable that the polymer includes a nitrilegroup-containing monomer unit. The proportion constituted by the nitrilegroup-containing monomer unit when the amount of all repeating unitsincluded in the polymer in the positive electrode mixed material layeris taken to be 100 mass % is preferably 35 mass % or more, morepreferably 45 mass % or more, and even more preferably 55 mass % ormore, and is preferably 75 mass % or less, and more preferably 70 mass %or less. The peel strength of the electrode can be further improved whenthe proportion constituted by the nitrile group-containing monomer unitin the polymer contained in the positive electrode mixed material layeris 35 mass % or more. On the other hand, a high level of safety of anelectrochemical device can sufficiently be ensured as well as furtherimproving the peel strength of the electrode when the proportionconstituted by the nitrile group-containing monomer unit in the polymercontained in the positive electrode mixed material layer is 75 mass % orless.

Total Proportion of Diene Monomer Unit and Nitrile Group-ContainingMonomer Unit

When the amount of all repeating units included in the polymer is takento be 100 mass %, the proportion constituted by the diene monomer unitand the proportion constituted by the nitrile group-containing monomerunit are, in total, required to be not less than 10 mass % and not morethan 80 mass % as previously described, are preferably 20 mass % ormore, and more preferably 30 mass % or more, and are preferably 75 mass% or less, and more preferably 70 mass % or less. The peel strength ofthe electrode decreases if the total proportion constituted by the dienemonomer unit and the nitrile group-containing monomer unit in thepolymer is less than 10 mass %. On the other hand, the polymeraggregates in the electrode mixed material layer if the total proportionconstituted by the diene monomer unit and the nitrile group-containingmonomer unit in the polymer is more than 80 mass %. As a consequence,the peel strength of the electrode decreases, and safety of anelectrochemical device cannot sufficiently be ensured.

In a case in which the presently disclosed electrode for anelectrochemical device is a positive electrode for an electrochemicaldevice, the proportion constituted by the diene monomer unit and theproportion constituted by the nitrile group-containing monomer unit whenthe amount of all repeating units included in the polymer is taken to be100 mass % are, in total, preferably 35 mass % or more, more preferably45 mass % or more, and even more preferably 55 mass % or more, and arepreferably 75 mass % or less, more preferably 70 mass % or less, andeven more preferably 65 mass % or less. The peel strength of theelectrode can be further improved when the total proportion constitutedby the diene monomer unit and the nitrile group-containing monomer unitin the polymer contained in the positive electrode mixed material layeris 35 mass % or more. On the other hand, a high level of safety of anelectrochemical device can sufficiently be ensured as well as furtherimproving the peel strength of the electrode when the total proportionconstituted by the diene monomer unit and the nitrile group-containingmonomer unit in the polymer contained in the positive electrode mixedmaterial layer is 75 mass % or less.

Moreover, in a case in which the presently disclosed electrode for anelectrochemical device is a negative electrode for an electrochemicaldevice, the proportion constituted by the diene monomer unit and theproportion constituted by the nitrile group-containing monomer unit whenthe amount of all repeating units included in the polymer is taken to be100 mass % are, in total, preferably 20 mass % or more, more preferably25 mass % or more, and even more preferably 30 mass % or more, and arepreferably 65 mass % or less, more preferably 50 mass % or less, andeven more preferably 40 mass % or less. The peel strength of theelectrode can be further improved when the total proportion constitutedby the diene monomer unit and the nitrile group-containing monomer unitin the polymer contained in the negative electrode mixed material layeris 20 mass % or more. On the other hand, a high level of safety of anelectrochemical device can sufficiently be ensured as well as furtherimproving the peel strength of the electrode when the total proportionconstituted by the diene monomer unit and the nitrile group-containingmonomer unit in the polymer contained in the negative electrode mixedmaterial layer is 65 mass % or less.

[Other repeating units]

Examples of the other monomer units included in the polymer include, butare not specifically limited to, an alkylene structural unit, an acidicgroup-containing monomer unit, a (meth)acrylic acid ester monomer unit,a (meth)acrylamide monomer unit, and an aromatic vinyl monomer unit.

In the present disclosure, “(meth)acryl” is used to indicate “acryl”and/or “methacryl”.

Alkylene Structural Unit

The alkylene structural unit is a repeating unit composed only of analkylene structure represented by a general formula —C_(n)H_(2n)— (n isan integer of 2 or more).

The alkylene structural unit may be linear or branched, but ispreferably linear. In other words, the alkylene structural unit ispreferably a linear alkylene structural unit. The carbon number of thealkylene structural unit is preferably 4 or more (i.e., n in thepreceding general formula is preferably an integer of 4 or more).

The method by which the alkylene structural unit is introduced into thepolymer is not specifically limited and may, for example, be thefollowing method (1) or (2).

(1) A method in which a polymer including an aliphatic conjugated dienemonomer unit is produced from a monomer composition containing analiphatic conjugated diene monomer and then the polymer is hydrogenatedto convert the aliphatic conjugated diene monomer unit to an alkylenestructural unit

(2) A method in which a polymer is produced from a monomer compositioncontaining a 1-olefin monomer

Of these methods, method (1) is preferable in terms of ease ofproduction of the polymer.

Examples of aliphatic conjugated diene monomers that can form analiphatic conjugated diene monomer unit that is convertible to analkylene structural unit include those described above in the “Dienemonomer unit and nitrile group-containing monomer unit” section. Ofthese aliphatic conjugated diene monomers, 1,3-butadiene is preferable.In other words, the alkylene structural unit is preferably a structuralunit obtained through hydrogenation of an aliphatic conjugated dienemonomer unit (i.e., is preferably a hydrogenated aliphatic conjugateddiene unit), and is more preferably a structural unit obtained throughhydrogenation of a 1,3-butadiene unit (i.e., is more preferably ahydrogenated 1,3-butadiene unit).

Examples of the 1-olefin monomer include ethylene, propylene, 1-butene,and 1-hexene.

One of these aliphatic conjugated diene monomers or 1-olefin monomersmay be used individually, or two or more of these aliphatic conjugateddiene monomers or 1-olefin monomers may be used in combination.

Acidic Group-Containing Monomer Unit

Examples of acidic group-containing monomers that can form an acidicgroup-containing monomer unit include carboxy group-containing monomers,sulfo group-containing monomers, and phosphate group-containingmonomers. These carboxy group-containing monomers, sulfogroup-containing monomers, and phosphate group-containing monomers maybe any of those described in JP 2017-069108 A. Of these acidicgroup-containing monomers, carboxy group-containing monomers arepreferable, and methacrylic acid and itaconic acid are more preferable.

One acidic group-containing monomer may be used individually, or two ormore acidic group-containing monomers may be used in combination.

(Meth)Acrylic Acid Ester Monomer Unit

Examples of (meth)acrylic acid ester monomers that can form a(meth)acrylic acid ester monomer unit include those described in JP2017-050112 A. Of these (meth)acrylic acid ester monomers,2-hydroxyethyl acrylate is preferable.

One (meth)acrylic acid ester monomer may be used individually, or two ormore (meth)acrylic acid ester monomers may be used in combination.

(Meth)Acrylamide Monomer Unit

Examples of (meth)acrylamide monomers that can form a (meth)acrylamidemonomer unit include acrylamide and methacrylamide. Of these(meth)acrylamide monomers, acrylamide is preferable.

One of acrylamide and methacrylamide may be used individually, or bothof acrylamide and methacrylamide may be used in combination.

Aromatic Vinyl Monomer

Examples of aromatic vinyl monomers that can form an aromatic vinylmonomer unit include styrene, α-methylstyrene, vinyltoluene, anddivinylbenzene. Of these aromatic vinyl monomers, styrene is preferable.

One aromatic vinyl monomer may be used individually, or two or morearomatic vinyl monomers may be used in combination.

Proportion of Other Repeating Units

The proportion constituted by other repeating units when the amount ofall repeating units included in the polymer is taken to be 100 mass % isnot less than 20 mass % and not more than 80 mass %, and is preferably30 mass % or more.

[Production Method]

No specific limitations are placed on the method by which the polymerserving as the binder described above is produced. The polymer servingas the binder can be produced by, for example, polymerizing a monomercomposition containing the monomers set forth above and subsequentlyperforming hydrogenation as necessary.

The fractional content of each monomer in the monomer composition usedto produce the polymer can be set in accordance with the fractionalcontent of each repeating unit in the polymer.

The polymerization method is not specifically limited and may, forexample, be any of solution polymerization, suspension polymerization,bulk polymerization, and emulsion polymerization. Also, anypolymerization reaction can be used, such as ionic polymerization,radical polymerization, or living radical polymerization.

The method of hydrogenation of the polymer is also not specificallylimited and may be a typical method using a catalyst (for example, referto WO 2012/165120 A1, WO 2013/080989 A1, and JP 2013-8485 A).

[Glass-Transition Temperature]

The glass-transition temperature of the polymer serving as the binderthat is produced as set forth above is preferably −30° C. or higher,more preferably −25° C. or higher, even more preferably −20° C. orhigher, and particularly preferably −10° C. or higher, and is preferably100° C. or lower, more preferably 70° C. or lower, even more preferably50° C. or lower, and particularly preferably 30° C. or lower. The peelstrength of the electrode can be further increased while also improvinghigh-temperature storage characteristics of an electrochemical devicewhen the glass-transition temperature of the polymer is −30° C. orhigher. On the other hand, the peel strength of the electrode can befurther increased while also sufficiently ensuring a high level ofsafety of an electrochemical device when the glass-transitiontemperature of the polymer is 100° C. or lower.

[Content]

The amount of the polymer serving as the binder that is contained in theelectrode mixed material layer per 100 parts by mass of the electrodeactive material is preferably 0.1 parts by mass or more, more preferably0.5 parts by mass or more, even more preferably 0.6 parts by mass ormore, and particularly preferably 0.8 parts by mass or more, and ispreferably 10 parts by mass or less, more preferably 7 parts by mass orless, even more preferably 5 parts by mass or less, and particularlypreferably 3 parts by mass or less. The peel strength of the electrodecan be further improved when the content of the polymer in the electrodemixed material layer is 0.1 parts by mass or more per 100 parts by massof the electrode active material, whereas rate characteristics of anelectrochemical device can be enhanced when the content of the polymerin the electrode mixed material layer is 10 parts by mass or less per100 parts by mass of the electrode active material.

<<Foaming Agent>>

The foaming agent can be a component that releases an incombustible gassuch as nitrogen, carbon dioxide, ammonia, or water vapor throughthermal decomposition.

[Foaming Temperature]

The foaming temperature of the foaming agent is preferably 100° C. orhigher, more preferably 140° C. or higher, even more preferably 150° C.or higher, particularly preferably 180° C. or higher, and mostpreferably 200° C. or higher, and is preferably 350° C. or lower, morepreferably 300° C. or lower, even more preferably 280° C. or lower, andparticularly preferably 250° C. or lower. A foaming temperature of 100°C. or higher for the foaming agent can inhibit unexpected foaming duringnormal operation or storage of an electrochemical device and can ensurerate characteristics and high-temperature storage characteristics of theelectrochemical device. On the other hand, a foaming temperature of 350°C. or lower for the foaming agent enables appropriate foaming during arise in internal temperature of an electrochemical device caused by aninternal short circuit or the like and can sufficiently ensure a highlevel of safety of the electrochemical device.

[Type]

The foaming agent may be either an organic foaming agent or an inorganicfoaming agent without any specific limitations.

The organic foaming agent is preferably an organic foaming agent(organonitrogen foaming agent) that releases nitrogen as anincombustible gas from a viewpoint of enhancing rate characteristics andsufficiently ensuring a high level of safety of an electrochemicaldevice. Examples of organonitrogen foaming agents include guanidinecompounds (guanidine nitrate, nitroguanidine, aminoguanidine nitrate,etc.), azo compounds (azodicarbonamide, azobisisobutyronitrile, etc.),triazine compounds (melamine, ammeline, ammelide, melamine cyanurate,trihydrazine triazine (1,3,5-triazine-2,4,6(1H,3H,5H)-trionetrihydrazone), etc.), hydrazide compounds (oxybis(benzenesulfonylhydrazide), p-toluenesulfonyl hydrazide, etc.), hydrazo compounds(hydrazodicarbonamide, p-toluenesulfonyl semicarbazide, etc.),nitroamine compounds (dinitrosopentamethylenetetramine,trimethylenetrinitramine, etc.), tetrazole compounds (5-aminotetrazole,5-phenyltetrazole, etc.), and bitetrazole compounds (5,5′-bitetrazolediammonium, bitetrazole piperazine, etc.).

Examples of inorganic foaming agents include hydrogen carbonates such assodium hydrogen carbonate.

One of these foaming agents may be used individually, or two or more ofthese foaming agents may be used in combination in a freely selectedratio.

Of these foaming agents, azo compounds, triazine compounds, hydrazidecompounds, hydrazo compounds, and hydrogen carbonate compounds arepreferable, triazine compounds and azo compounds are more preferable,and melamine cyanurate and azodicarbonamide are even more preferablefrom a viewpoint of further enhancing rate characteristics and moresufficiently ensuring a high level of safety of an electrochemicaldevice.

[Content]

The amount of the foaming agent that is contained in the electrode mixedmaterial layer per 100 parts by mass of the electrode active material ispreferably 0.01 parts by mass or more, more preferably 0.05 parts bymass or more, even more preferably 0.1 parts by mass or more,particularly preferably 0.3 parts by mass or more, and most preferably0.5 parts by mass or more, and is preferably 10 parts by mass or less,more preferably 5 parts by mass or less, even more preferably 3 parts bymass or less, particularly preferably 2 parts by mass or less, and mostpreferably 1 part by mass or less. The peel strength of the electrodecan be further increased while also sufficiently ensuring a high levelof safety of an electrochemical device when the content of the foamingagent in the electrode mixed material layer is 0.01 parts by mass ormore per 100 parts by mass of the electrode active material. On theother hand, the peel strength of the electrode can be further increasedwhile also improving rate characteristics of an electrochemical devicewhen the content of the foaming agent in the electrode mixed materiallayer is 10 parts by mass or less per 100 parts by mass of the electrodeactive material.

The amount of the foaming agent that is contained in the electrode mixedmaterial layer per 100 parts by mass of the polymer serving as thebinder is preferably 10 parts by mass or more, more preferably 20 partsby mass or more, and even more preferably 30 parts by mass or more, andis preferably 600 parts by mass or less, more preferably 300 parts bymass or less, and even more preferably 150 parts by mass or less. Goodinteraction between the polymer serving as the binder and the foamingagent can be achieved and the peel strength of the electrode can befurther increased when the content of the foaming agent in the electrodemixed material layer is not less than 10 parts by mass and not more than600 parts by mass per 100 parts by mass of the polymer serving as thebinder.

<<Other Components>>

In addition to the electrode active material, binder, and foaming agentset forth above, the electrode mixed material layer may optionallycontain other binders having different chemical compositions to thebinder described above and known additives that can be added inelectrode mixed material layers such as conductive materials, wettingagents, viscosity modifiers, and additives for electrolyte solution. Oneof these other components may be used individually, or two or more ofthese other components may be used in combination. The content of theabove-described additives in the electrode mixed material layer may, forexample, be set as 10 parts by mass or less per 100 parts by mass of thebinder, or as 5 parts by mass or less per 100 parts by mass of thebinder.

<Current collector>

The current collector may be a material having electrical conductivityand electrochemical durability that is selected in accordance with thetype of electrochemical device. The current collector of an electrodefor a lithium ion secondary battery, for example, may be a currentcollector made from iron, copper, aluminum, nickel, stainless steel,titanium, tantalum, gold, platinum, or the like. Of these examples,copper foil is particularly preferable as a current collector used for anegative electrode. On the other hand, aluminum foil is particularlypreferable as a current collector used for a positive electrode. One ofthese materials may be used individually, or two or more of thesematerials may be used in combination in a freely selected ratio.

A known layer such as a conductive adhesive layer may be disposed at thesurface of the current collector. In other words, the current collectormay be a conductive adhesive layer-equipped current collector thatincludes a conductive adhesive layer at the surface.

<Volume Resistivity of Electrode Laminate>

In the presently disclosed electrode for an electrochemical device, thevolume resistivity R_(A) (25° C.) of an electrode laminate in which theelectrode mixed material layer and current collector set forth above arestacked is required to be not less than 0.1 Ω·cm and not more than 200Ω·cm from a viewpoint of sufficiently ensuring device characteristics(rate characteristics, etc.) of an electrochemical device.

In a case in which the presently disclosed electrode for anelectrochemical device is a positive electrode for an electrochemicaldevice, the volume resistivity R_(A) (25° C.) of the electrode laminateis preferably 10 Ω·cm or more, more preferably 20 Ω·cm or more, and evenmore preferably 30 Ω·cm or more, and is preferably 180 Ω·cm or less,more preferably 150 Ω·cm or less, and even more preferably 100 S/cm orless.

Moreover, in a case in which the presently disclosed electrode for anelectrochemical device is a negative electrode for an electrochemicaldevice, the volume resistivity R_(A) (25° C.) of the electrode laminateis preferably 0.2 Ω·cm or more, more preferably 0.3 Ω·cm or more, evenmore preferably 0.4 Ω·cm or more, and particularly preferably 0.8 Ω·cmor more, and is preferably 50 Ω·cm or less, more preferably 10 Ω·cm orless, and even more preferably 3 Ω·cm or less.

The volume resistivity R_(A) (25° C.) of the electrode laminate can beadjusted by, for example, altering the type and amount of the electrodeactive material, binder, conductive material, and foaming agentcontained in the electrode mixed material layer and the productionconditions (stirring time, solid content concentration during stirring,stirring rate, etc.) of a slurry composition for the electrode mixedmaterial layer.

A ratio of the volume resistivity R_(B) (350° C.) of the electrodelaminate relative to the volume resistivity R_(A) (25° C.) of theelectrode laminate is required to be 10 or more from a viewpoint ofensuring a high level of safety of an electrochemical device, and ispreferably 20 or more, more preferably 30 or more, even more preferably100 or more, and particularly preferably 200 or more. Although nospecific limitations are placed on the upper limit for the ratio of thevolume resistivity R_(B) (350° C.) relative to the volume resistivityR_(A) (25° C.), this ratio is normally 10,000 or less.

Note that the ratio of the volume resistivity R_(B) (350° C.) of theelectrode laminate relative to the volume resistivity R_(A) (25° C.) ofthe electrode laminate can be increased, in particular, by increasingthe volume resistivity R_(B) (350° C.). The volume resistivity R_(B)(350° C.) can be increased through selection of the type of binder orfoaming agent or by increasing the amount of the foaming agent, forexample.

<Other Layers>

Examples of other layers that may optionally be disposed at the surfaceof the electrode (particularly the surface at the electrode mixedmaterial layer-side) include, but are not specifically limited to, aknown heat-resistant layer that is provided with the aim of improvingheat resistance and a known adhesive layer that is provided with the aimof improving adhesiveness with another battery component such as aseparator.

(Method of Producing Electrode for Electrochemical Device)

The presently disclosed electrode for an electrochemical device setforth above can, for example, be produced using the presently disclosedmethod of producing an electrode for an electrochemical device.

The presently disclosed method of producing an electrode includes a stepof applying a slurry composition for an electrode mixed material layercontaining an electrode active material, a binder, a foaming agent, anda solvent onto a current collector (application step) and a step ofdrying the slurry composition for an electrode mixed material layerapplied on the current collector at a temperature of not lower than 50°C. and not higher than 130° C. to form an electrode mixed material layer(electrode mixed material layer formation step).

Water or an organic solvent can be used as the solvent in the slurrycomposition for an electrode mixed material layer without any specificlimitations. Examples of organic solvents that can be used includeacetonitrile, N-methyl-2-pyrrolidone, tetrahydrofuran, acetone,acetylpyridine, cyclopentanone, dimethylformamide, dimethyl sulfoxide,methylformamide, methyl ethyl ketone, furfural, ethylenediamine,dimethylbenzene (xylene), methylbenzene (toluene), cyclopentyl methylether, and isopropyl alcohol.

One of these solvents may be used individually, or two or more of thesesolvents may be used as a mixture in a freely selected mixing ratio.

The slurry composition for an electrode mixed material layer may alsocontain any components other than those described above. For example,the slurry composition for an electrode mixed material layer can containany of the other components that can be contained in the electrode mixedmaterial layer as previously described.

<Application Step>

The slurry composition for an electrode mixed material layer can beapplied onto the current collector by any commonly known method withoutany specific limitations. Specific examples of application methods thatcan be used include doctor blading, dip coating, reverse roll coating,direct roll coating, gravure coating, extrusion coating, and brushcoating. During application, the slurry composition may be applied ontoone side or both sides of the current collector. The thickness of theslurry coating on the current collector after application but beforedrying may be set as appropriate in accordance with the desiredelectrode mixed material layer thickness.

<Electrode Mixed Material Layer Formation Step>

The slurry composition for an electrode mixed material layer on thecurrent collector may be dried by any commonly known method without anyspecific limitations. Examples of drying methods that can be usedinclude drying by warm, hot, or low-humidity air; drying in a vacuum;and drying by irradiation with infrared light, electron beams, or thelike. In the presently disclosed method of producing an electrode, theatmosphere temperature during drying of the slurry composition for anelectrode mixed material layer (i.e., the drying temperature) is notlower than 50° C. and not higher than 130° C., and is preferably notlower than 70° C. and not higher than 100° C. A drying temperature ofnot lower than 50° C. and not higher than 130° C. can ensure dryingefficiency while also inhibiting decomposition of the foaming agent.

A pressing process or the like may optionally be performed after thedrying to obtain an electrode including an electrode mixed materiallayer on the current collector.

(Electrochemical Device)

The presently disclosed electrochemical device may, for example, be asecondary battery such as a lithium ion secondary battery, a primarybattery such as a lithium battery or a lithium-air battery, or acapacitor such as an electric double-layer capacitor or a lithium ioncapacitor, but is not specifically limited thereto, and is preferably asecondary battery (particularly a lithium ion secondary battery) forwhich there has been increasing need for a balance of high capacity andsafety in recent years. The presently disclosed electrochemical deviceincludes the presently disclosed electrode. As a result of including thepresently disclosed electrode, the presently disclosed electrochemicaldevice inhibits thermal runaway and maintains a high level of safety.

Although the following describes, as one example, a case in which theelectrochemical device is a lithium ion secondary battery, the presentlydisclosed electrochemical device is not limited to the followingexample. A lithium ion secondary battery corresponding to the presentlydisclosed electrochemical device normally includes electrodes (positiveelectrode and negative electrode), an electrolyte solution, and aseparator, wherein the presently disclosed electrode for anelectrochemical device is used for at least one of the positiveelectrode and the negative electrode.

<Electrodes>

A known electrode can be used without any specific limitations as anelectrode other than the presently disclosed electrode for anelectrochemical device set forth above that may be used in the lithiumion secondary battery corresponding to the presently disclosedelectrochemical device. Specifically, an electrode obtained by formingan electrode mixed material layer on a current collector by a knownproduction method may be used as an electrode other than the electrodefor an electrochemical device set forth above.

<Separator>

Examples of separators that can be used include, but are notspecifically limited to, those described in JP 2012-204303 A. Of theseseparators, a microporous membrane made of polyolefinic (polyethylene,polypropylene, polybutene, or polyvinyl chloride) resin is preferredsince such a membrane can reduce the total thickness of the separator,which increases the ratio of electrode active material in the lithiumion secondary battery, and consequently increases the capacity pervolume.

<Electrolyte solution>

The electrolyte solution is normally an organic electrolyte solutionobtained by dissolving a supporting electrolyte in an organic solvent.The supporting electrolyte of the lithium ion secondary battery may, forexample, be a lithium salt. Examples of lithium salts that can be usedinclude LiPF₆, LiAsF₆, LiBF₄, LiSbF₆, LiAlCl₄, LiClO₄, CF₃SO₃Li,C₄F₉SO₃Li, CF₃COOLi, (CF₃CO)₂NLi, (CF₃SO₂)₂NLi, and (C₂F₅SO₂)NLi. Ofthese lithium salts, LiPF₆, LiClO₄, and CF₃SO₃Li are preferable, andLiPF₆ is particularly preferable as these lithium salts readily dissolvein solvents and exhibit a high degree of dissociation. One electrolytemay be used individually, or two or more electrolytes may be used incombination in a freely selected ratio. In general, lithium ionconductivity tends to increase when a supporting electrolyte having ahigh degree of dissociation is used. Therefore, lithium ion conductivitycan be adjusted through the type of supporting electrolyte that is used.

The organic solvent used in the electrolyte solution is not specificallylimited so long as the supporting electrolyte can dissolve therein.Examples of suitable organic solvents include carbonates such asdimethyl carbonate (DMC), ethylene carbonate (EC), diethyl carbonate(DEC), propylene carbonate (PC), butylene carbonate (BC), and ethylmethyl carbonate (EMC); esters such as y-butyrolactone and methylformate; ethers such as 1,2-dimethoxyethane and tetrahydrofuran; andsulfur-containing compounds such as sulfolane and dimethyl sulfoxide.Furthermore, a mixture of such solvents may be used. Of these solvents,carbonates are preferable due to having a high permittivity and a widestable potential region, and a mixture of ethylene carbonate and ethylmethyl carbonate is more preferable.

Moreover, a known additive such as vinylene carbonate (VC),fluoroethylene carbonate (FEC), or ethyl methyl sulfone may be added tothe electrolyte solution.

<Production Method of Lithium Ion Secondary Battery>

The lithium ion secondary battery corresponding to the presentlydisclosed electrochemical device can be produced by, for example,stacking the positive electrode and the negative electrode with theseparator interposed in-between, performing rolling, folding, or thelike of the resultant laminate as necessary to place the laminate in abattery container, injecting the electrolyte solution into the batterycontainer, and sealing the battery container. Note that at least one ofthe positive electrode and the negative electrode is the presentlydisclosed electrode for an electrochemical device. In order to preventpressure increase inside the battery and occurrence of overcharging oroverdischarging, an expanded metal; an overcurrent preventing devicesuch as a fuse or a PTC device; or a lead plate may be provided in thebattery container as necessary. The shape of the battery may, forexample, be a coin type, a button type, a sheet type, a cylinder type, aprismatic type, or a flat type.

EXAMPLES

The following provides a more specific description of the presentdisclosure based on examples. However, the present disclosure is notlimited to the following examples. In the following description, “%” and“parts” used in expressing quantities are by mass, unless otherwisespecified.

Moreover, in the case of a polymer that is produced throughcopolymerization of a plurality of types of monomers, the proportionconstituted by a monomer unit in the polymer that is formed throughpolymerization of a given monomer is normally, unless otherwisespecified, the same as the ratio (charging ratio) of the given monomeramong all monomers used in polymerization of the polymer.

In the examples and comparative examples, the following methods wereused to evaluate the foaming temperature of a foaming agent, theglass-transition temperature of a polymer, the content of a foamingagent in an electrode mixed material layer, the volume resistivity of anelectrode laminate, the peel strength of an electrode, and the ratecharacteristics, high-temperature storage characteristics, and safety ofa lithium ion secondary battery.

<Foaming Temperature>

In thermogravimetric analysis using a thermogravimetric analyzer (TG8110produced by Rigaku Corporation), the mass of a foaming agent wasmeasured while heating the foaming agent from 25° C. to 500° C. at aheating rate of 20° C./min in an air atmosphere, and the temperature atwhich the measured mass reached 95% of the mass at the start ofmeasurement (25° C.) (i.e., the 5% mass loss temperature) was taken tobe the foaming temperature of the foaming agent.

<Glass-Transition Temperature>

Each polymer used as a binder was taken as a measurement sample. Adifferential scanning calorimetry (DSC) curve was obtained using adifferential scanning calorimeter (EXSTAR DSC6220 produced by SIINanoTechnology Inc.) by weighing 10 mg of the measurement sample into analuminum pan and then performing measurement in a measurementtemperature range of −100° C. to 500° C. at a heating rate of 10° C./minunder conditions stipulated by JIS Z 8703 and with an empty aluminum panas a reference. In the heating process, the glass-transition temperature(° C.) was determined as an intersection point of a base line directlybefore a heat absorption peak on the DSC curve at which a differentialsignal (DDSC) reached 0.05 mW/min/mg or more and a tangent to the DSCcurve at a first inflection point to appear after the heat absorptionpeak.

<Content of Foaming Agent>

<<Organonitrogen Foaming Agent>>

A sample was obtained by scraping an electrode mixed material layer froma current collector of a produced electrode. Next, 1 g of the sample wasimmersed in 50 g of 25° C. tetrahydrofuran for 24 hours. Afterimmersion, the sample was separated using a membrane filter (H050A090Cproduced by Advantec Co., Ltd.), and solid matter on the filter wascollected. The nitrogen content in the collected solid matter wasmeasured by the modified Dumas method, and then the content (parts bymass per 100 parts by mass of electrode active material) of anorganonitrogen foaming agent in the electrode mixed material layer wascalculated based on the determined nitrogen content.

Note that the type of organonitrogen foaming agent contained in anelectrode mixed material layer can be identified by a known analyticaltechnique (for example, gas chromatography, high-performance liquidchromatography, or NMR). Therefore, in a situation in which the type oforganonitrogen foaming agent contained in an electrode mixed materiallayer is unknown, the type of organonitrogen foaming agent can beidentified by any of the aforementioned techniques, and then themolecular weight of the identified organonitrogen foaming agent can beused to calculate the content of the organonitrogen foaming agent in theelectrode mixed material layer.

<<Inorganic Foaming Agent>>

A sample was obtained by scraping an electrode mixed material layer froma current collector of a produced electrode. A temperature-programmeddesorption gas analyzer (TDS1200 produced by ESCO, Ltd.) was used toheat 5 mg of the sample to 200° C. at a heating rate of 10° C./min. Thereleased amount of carbon dioxide gas was calculated from the number ofmolecules detected for a mass number of 44. The content (parts by massper 100 parts by mass of electrode active material) of an inorganicfoaming agent in the electrode mixed material layer was then calculatedbased on the released amount of carbon dioxide gas that had beendetermined.

Note that the type of inorganic foaming agent contained in an electrodemixed material layer can be identified by a known analytical technique(for example, gas chromatography, high-performance liquidchromatography, or NMR). Therefore, in a situation in which the type ofinorganic foaming agent contained in an electrode mixed material layeris unknown, the type of inorganic foaming agent can be identified by anyof the aforementioned techniques, and then the molecular weight of theidentified inorganic foaming agent can be used to calculate the contentof the inorganic foaming agent in the electrode mixed material layer.

<Volume Resistivity>

A circle of 1.2 cm in diameter (circle area taken to be S (cm²)) waspunched out from a produced electrode (electrode laminate) as a testspecimen. The thickness d (cm) of the test specimen was accuratelymeasured. The test specimen was then sandwiched in a load cell of atensile compression tester (SV-301NA produced by Imada-SS Corporation)and was pressed with a pressure of 20 MPa. A two-terminal type clip wasconnected to the load cell and a measurement cable was connected to apolarization system (HSV-110 produced by Hokuto Denko Corporation).Current was passed through the load cell for 1 minute with a set currentI of 10 mA in chronopotentiometry mode, and the voltage (V) at that timewas measured. The volume resistivity (Ω·cm) was calculated as(V/I)×(S/d). In this measurement, the volume resistivity measured in anenvironment having a temperature of 25° C. and a dew point of −40° C.was taken to be R_(A) (Ω·cm). Moreover, the volume resistivity measuredin an environment having a temperature of 25° C. and a dew point of −40°C. after the electrode had been heated in a 350° C. thermostatic chamberfor 30 minutes was taken to be R_(B) (Ω·cm). The value of R_(B)/R_(A)was calculated. A smaller volume resistivity R_(A) (25° C.) for theelectrode laminate indicates that the electrode has better electricalconductivity and contributes to improving lithium ion secondary batteryrate characteristics. Moreover, a larger R_(B)/R_(A) value for theelectrode laminate indicates that the electrode contributes to improvinglithium ion secondary battery safety.

In the present disclosure, in a case in which an electrode for anelectrochemical device includes another layer at the surface at theelectrode mixed material layer-side, for example, the volume resistivityof the electrode laminate is taken to be that measured either beforestacking the other layer or after peeling off the other layer.

<Peel Strength>

<<Negative Electrode>>

A rectangle of 100 mm in length and 10 mm in width was cut out from aproduced negative electrode as a test specimen. Next, the test specimenwas placed with the surface at which the negative electrode mixedmaterial layer was located facing downward and the surface of thenegative electrode mixed material layer was affixed to the surface of asubstrate made from SUS using cellophane tape (tape prescribed by JISZ1522). Thereafter, one end of the current collector was pulled in avertical direction at a pulling speed of 50 mm/min to peel off thecurrent collector, and the stress (N/m) during this peeling wasmeasured. This measurement was performed three times in total. Theaverage value of these measurements was determined as the peel strengthof the negative electrode and was evaluated by the following standard. Alarger value for the peel strength of the negative electrode indicatesstronger close adherence between the negative electrode mixed materiallayer and the current collector and better adhesiveness of the negativeelectrode mixed material layer.

A: Negative electrode peel strength of 3.5 N/m or more

B: Negative electrode peel strength of not less than 3.0 N/m and lessthan 3.5 N/m

C: Negative electrode peel strength of not less than 2.5 N/m and lessthan 3.0 N/m D: Negative electrode peel strength of not less than 1.5N/m and less than 2.5 N/m

E: Negative electrode peel strength of less than 1.5 N/m

<<Positive Electrode>>

A rectangle of 100 mm in length and 10 mm in width was cut out from aproduced positive electrode as a test specimen. Next, the test specimenwas placed with the surface at which the positive electrode mixedmaterial layer was located facing downward and the surface of thepositive electrode mixed material layer was affixed to the surface of asubstrate made from SUS using cellophane tape (tape prescribed by JISZ1522). Thereafter, one end of the current collector was pulled in avertical direction at a pulling speed of 50 mm/min to peel off thecurrent collector, and the stress (N/m) during this peeling wasmeasured. This measurement was performed three times in total. Theaverage value of these measurements was determined as the peel strengthof the positive electrode and was evaluated by the following standard. Alarger value for the peel strength of the positive electrode indicatesstronger close adherence between the positive electrode mixed materiallayer and the current collector and better adhesiveness of the positiveelectrode mixed material layer.

A: Positive electrode peel strength of 50.0 N/m or more

B: Positive electrode peel strength of not less than 40.0 N/m and lessthan 50.0 N/m

C: Positive electrode peel strength of not less than 30.0 N/m and lessthan 40.0 N/m

D: Positive electrode peel strength of not less than 20.0 N/m and lessthan 30.0 N/m

E: Positive electrode peel strength of less than 20.0 N/m

<Rate Characteristics>

A produced lithium ion secondary battery was left at rest for 5 hours ata temperature of 25° C. after being filled with electrolyte solution.Next, the lithium ion secondary battery was charged to a cell voltage of3.65 V by a 0.2 C constant-current method at a temperature of 25° C.,and was then subjected to aging treatment for 12 hours at a temperatureof 60° C. The lithium ion secondary battery was subsequently dischargedto a cell voltage of 3.00 V by a 0.2 C constant-current method at atemperature of 25° C. Thereafter, CC-CV charging of the lithium ionsecondary battery was performed with a 0.2 C constant current (upperlimit cell voltage: 4.35 V) and CC discharging of the lithium ionsecondary battery was performed to a cell voltage of 3.00 V with a 0.2 Cconstant current. This charging and discharging at 0.2 C was repeatedthree times.

Next, the lithium ion secondary battery was subjected to 0.2 Cconstant-current charging and discharging between cell voltages of 4.35V and 3.00 V in an environment having a temperature of 25° C., and thedischarge capacity at that time was defined as C0. Thereafter, thelithium ion secondary battery was CC-CV charged with a 0.2 C constantcurrent in the same manner and was discharged to 2.5 V with a 0.5 Cconstant current in an environment having a temperature of −10° C. Thedischarge capacity at that time was defined as C1. The capacitymaintenance rate indicated by ΔC=(C1/C0)×100(%) was calculated as a ratecharacteristic and was evaluated by the following standard. A largervalue for the capacity maintenance rate AC indicates higher dischargecapacity at high current in a low temperature environment, and lowerinternal resistance.

A: Capacity maintenance rate ΔC of 70% or more

B: Capacity maintenance rate ΔC of not less than 65% and less than 70%

C: Capacity maintenance rate ΔC of not less than 60% and less than 65%

D: Capacity maintenance rate ΔC of less than 60%

<High-Temperature Storage Characteristics>

A produced lithium ion secondary battery was subjected to acharge/discharge operation of charging to 4.35 V by a constant-currentconstant-voltage (CC-CV) method with a charge rate of 0.2 C (cut-offcondition: 0.02 C) and discharging to 3.0 V by a constant-current (CC)method with a discharge rate of 0.2 C in a 25° C. atmosphere, and theinitial capacity C0′ was measured.

The lithium ion secondary battery was then charged to 4.35 V by aconstant-current constant-voltage (CC-CV) method with a charge rate of0.2 C (cut-off condition: 0.02 C). The lithium ion secondary battery wassubsequently stored in a 60° C. thermostatic tank for 14 days and wasthen left at rest for 2 hours in a 25° C. atmosphere. Next, the lithiumion secondary battery was discharged to 3.0 V by a constant-current (CC)method with a discharge rate of 0.2 C, and the post-high temperaturestorage capacity C1′ was measured. The capacity maintenance rateΔC′=(C1′/C0′)×100(%) was calculated and was evaluated by the followingstandard. A larger value for the capacity maintenance rate ΔC′ indicatesless reduction of discharge capacity due to high-temperature storage andbetter high-temperature storage characteristics.

A: Capacity maintenance rate ΔC′ of 90% or more

B: Capacity maintenance rate ΔC' of not less than 87% and less than 90%

C: Capacity maintenance rate ΔC′ of not less than 84% and less than 87%D: Capacity maintenance rate ΔC′ of less than 84%

<Safety>

A produced lithium ion secondary battery was charged to 4.35 V by aconstant-current constant-voltage (CC-CV) method with a charge rate of0.2 C (cut-off condition: 0.02 C) in a 25° C. atmosphere. Thereafter, aniron nail of 3 mm in diameter and 10 cm in length was pierced throughthe lithium ion secondary battery at a rate of 5 m/min in a roughlycentral location to induce a short circuit. A short circuit was inducedby the same operation for 5 lithium ion secondary batteries (testspecimens). The number of test specimens for which rupture and ignitiondid not occur was used to evaluate safety by the following standard.

A: Number of test specimens for which rupture and ignition do not occuris 4 or 5

B: Number of test specimens for which rupture and ignition do not occuris 3

C: Number of test specimens for which rupture and ignition do not occuris 2

D: Number of test specimens for which rupture and ignition do not occuris 1

E: Number of test specimens for which rupture and ignition do not occuris 0

(Production of Binder)

<Polymer A>

An autoclave equipped with a stirrer was charged with 240 parts ofdeionized water, 2.5 parts of sodium alkylbenzenesulfonate as anemulsifier, 60.0 parts of acrylonitrile as a nitrile group-containingmonomer, and 0.45 parts of t-dodecyl mercaptan as a chain transfer agentin this order, and the inside thereof was purged with nitrogen.Thereafter, 40.0 parts of 1,3-butadiene as a diene monomer was fed intothe autoclave under pressure, 0.25 parts of ammonium persulfate wasadded as a polymerization initiator, and a polymerization reaction wascarried out at a reaction temperature of 40° C. Through this reaction, awater dispersion of a copolymer of acrylonitrile and 1,3-butadiene wasobtained. The polymerization conversion rate was 85%.

Further deionized water was added to the resultant water dispersion ofthe copolymer so as to obtain a water dispersion that was adjusted to atotal solid content concentration of 12 mass %. A stirrer-equippedautoclave of 1 L in capacity was charged with 400 mL (total solidcontent: 48 g) of the obtained water dispersion, and then nitrogen gaswas passed for 10 minutes to remove dissolved oxygen in the dispersion.Thereafter, 75 mg of palladium acetate as a hydrogenation reactioncatalyst was dissolved in 180 mL of deionized water to which nitric acidhad been added in an amount of 4 molar equivalents of the palladium(Pd), and the resultant solution was added into the autoclave. Thesystem was purged twice with hydrogen gas, and then the contents of theautoclave were heated to 50° C. in a state in which the pressure wasincreased to 3 MPa with hydrogen gas, and a hydrogenation reaction(first stage hydrogenation reaction) was performed for 6 hours.

The autoclave was subsequently returned to atmospheric pressure. Then,25 mg of palladium acetate as a hydrogenation reaction catalyst wasdissolved in 60 mL of deionized water to which nitric acid had beenadded in an amount of 4 molar equivalents of the Pd, and the resultantsolution was added into the autoclave. The system was purged twice withhydrogen gas, and then the contents of the autoclave were heated to 50°C. in a state in which the pressure was increased to 3 MPa with hydrogengas, and a hydrogenation reaction (second stage hydrogenation reaction)was performed for 6 hours.

Next, the contents of the autoclave were returned to normal temperatureand the system was converted to a nitrogen atmosphere. Thereafter, thecontents were concentrated to a solid content concentration of 40% usingan evaporator to yield a water dispersion of a polymer. NMP was added tothe obtained water dispersion of the polymer so as to adjust the solidcontent concentration of the polymer to 7%. Water and excess NMP werethen removed by vacuum distillation at 90° C. to yield an NMP solution(solid content concentration: 8%) of a polymer A as a binder.

The iodine value of the obtained polymer A was measured. Note that theiodine value of the polymer was measured in accordance with JISK6235(2006) after causing coagulation of the polymer. It was confirmedthrough calculation from the obtained iodine value that the polymer Aincluded a 1,3-butadiene monomer unit as a diene monomer unit in aproportion of 1.5% and a hydrogenated 1,3-butadiene unit as an alkylenestructural unit in a proportion of 38.5%.

The polymer A had a glass-transition temperature of 7.8° C.

<Polymer B>

An NMP solution (solid content concentration: 8%) of a polymer B as abinder was obtained in the same way as the polymer A with the exceptionthat the amount of acrylonitrile as a nitrile group-containing monomerwas changed to 50.0 parts and the amount of 1,3-butadiene as a dienemonomer was changed to 50.0 parts.

The iodine value of the obtained polymer B was measured in the same wayas for the polymer A. It was confirmed through calculation from theobtained iodine value that the polymer B included a 1,3-butadienemonomer unit as a diene monomer unit in a proportion of 1.8% and ahydrogenated 1,3-butadiene unit as an alkylene structural unit in aproportion of 48.2%.

The Polymer B had a Glass-Transition Temperature of −12° C.

<Polymer C>

An NMP solution (solid content concentration: 8%) of a polymer C as abinder was obtained in the same way as the polymer A with the exceptionthat the amount of acrylonitrile as a nitrile group-containing monomerwas changed to 40.0 parts and the amount of 1,3-butadiene as a dienemonomer was changed to 60.0 parts.

The iodine value of the obtained polymer C was measured in the same wayas for the polymer A. It was confirmed through calculation from theobtained iodine value that the polymer C included a 1,3-butadienemonomer unit as a diene monomer unit in a proportion of 1.3% and ahydrogenated 1,3-butadiene unit as an alkylene structural unit in aproportion of 58.7%.

The polymer C had a glass-transition temperature of −29° C.

<Polymer D>

A reactor A having a mechanical stirrer and a condenser attached theretowas charged with 85 parts of deionized water and 0.2 parts of sodiumlinear alkylbenzenesulfonate in a nitrogen atmosphere. The contents ofthe reactor A were heated to 55° C. under stirring and 0.3 parts ofpotassium persulfate was added into the reactor A in the form of a 5.0%aqueous solution. Next, 62.0 parts of acrylonitrile as a nitrilegroup-containing monomer, 2.0 parts of methacrylic acid as an acidicgroup-containing monomer, 35.0 parts of 2-hydroxyethyl acrylate as a(meth)acrylic acid ester monomer, 1.0 parts of acrylamide as a(meth)acrylamide monomer, 0.6 parts of sodium linearalkylbenzenesulfonate, 0.035 parts of t-dodecyl mercaptan, 0.4 parts ofpolyoxyethylene lauryl ether, and 80 parts of deionized water were addedinto a vessel B with an attached mechanical stirrer in a nitrogenatmosphere, and were stirred and emulsified to produce a monomermixture. This monomer mixture was added into the reactor A at a constantrate over 5 hours while in a stirred and emulsified state, and areaction was carried out until the polymerization conversion ratereached 95% to yield a water dispersion of a copolymer. NMP was added tothe obtained water dispersion of the copolymer so as to adjust the solidcontent concentration of the copolymer to 7%. Water and excess NMP werethen removed by vacuum distillation at 90° C. to yield an NMP solution(solid content concentration: 8%) of a polymer D as a binder.

The polymer D had a glass-transition temperature of 66° C.

<Polymer E>

A 5 MPa pressure vessel equipped with a stirrer was charged with 64parts of styrene as an aromatic vinyl monomer, 32 parts of 1,3-butadieneas a diene monomer, 3 parts of itaconic acid as an acidicgroup-containing monomer, 1 part of 2-hydroxyethyl acrylate as a(meth)acrylic acid ester monomer, 0.3 parts of t-dodecyl mercaptan as amolecular weight modifier, 5 parts of sodium dodecylbenzenesulfonate asan emulsifier, 150 parts of deionized water as a solvent, and 1 part ofpotassium persulfate as a polymerization initiator. These materials weresufficiently stirred and were then heated to a temperature of 55° C. toinitiate polymerization. The reaction was terminated by cooling at thepoint at which the polymerization conversion rate reached 95.0%. Theresultant water dispersion of a copolymer was adjusted to pH 8 throughaddition of 5% sodium hydroxide aqueous solution. Unreacted monomer wassubsequently removed by thermal-vacuum distillation. Thereafter, coolingwas performed to a temperature of 30° C. or lower to obtain a waterdispersion of a polymer E as a binder.

The polymer E had a glass-transition temperature of 17° C.

<Polymer F>

A water dispersion of a polymer F as a binder was obtained in the sameway as the polymer E with the exception that the amount of styrene as anaromatic vinyl monomer was changed to 32 parts, the amount of1,3-butadiene as a diene monomer was changed to 37 parts, and 30 partsof methacrylic acid was used instead of itaconic acid as an acidicgroup-containing monomer.

The polymer F had a glass-transition temperature of 37° C.

<Polymer G>

A water dispersion of a polymer G as a binder was obtained in the sameway as the polymer E with the exception that the amount of styrene as anaromatic vinyl monomer was changed to 32 parts, the amount of1,3-butadiene as a diene monomer was changed to 54 parts, and 10 partsof acrylonitrile was additionally used as a nitrile group-containingmonomer.

The polymer G had a glass-transition temperature of −23° C.

Example 1

<Production of Positive Electrode>

A slurry composition for a positive electrode mixed material layer wasproduced by adding 100 parts of a lithium-containing complex oxide ofCo-Ni-Al (LiNi_(0.8)Co_(0.15)Al_(0.05)O₂) as a positive electrode activematerial, 2 parts of acetylene black (produced by Denki Kagaku KogyoKabushiki Kaisha; product name: HS-100) as a conductive material, 2parts in terms of solid content of the NMP solution of the polymer A asa binder, and 0.8 parts of azodicarbonamide (foaming temperature: 200°C.; average particle diameter: 5 μm; nitrogen content: 48 weight %) as afoaming agent into a planetary mixer, further adding NMP as a dispersionmedium to adjust the total solid content concentration to 67%, and thenmixing these materials.

A comma coater was then used to apply the obtained slurry compositionfor a positive electrode mixed material layer onto aluminum foil(thickness: 20 μm) serving as a current collector so as to have acoating weight of 20±0.5 mg/cm².

The slurry composition on the aluminum foil was dried by conveying thealuminum foil inside a 90° C. oven for 4 minutes and inside a 100° C.oven for 4 minutes at a speed of 300 mm/min, and in this manner apositive electrode web including a positive electrode mixed materiallayer formed on the current collector was obtained.

The positive electrode mixed material layer-side of the producedpositive electrode web was subsequently roll pressed in an environmenthaving a temperature of 25 ±3° C. and with a line pressure of 14 t(tons) to obtain a positive electrode having a positive electrode mixedmaterial layer density of 3.40 g/cm³. Thereafter, the positive electrodewas left for 1 week in an environment having a temperature of 25±3° C.and a relative humidity of 50±5%. The positive electrode that had beenleft was subsequently used to evaluate the content of foaming agent(organonitrogen foaming agent) in the positive electrode mixed materiallayer, the peel strength of the positive electrode, and the volumeresistivity of the positive electrode laminate (electrode laminate). Theresults are shown in Table 1.

<Production of Negative Electrode>

A planetary mixer was charged with 50 parts of artificial graphite(theoretical capacity: 360 mAh/g) and 50 parts of natural graphite(theoretical capacity: 360 mAh/g) as negative electrode activematerials, and 1 part in terms of solid content of carboxymethylcellulose as a thickener. These materials were diluted to a solidcontent concentration of 60% with deionized water and were then kneadedat a rotation speed of 45 rpm for 60 minutes. Next, 1 part in terms ofsolid content of the water dispersion of the polymer E was added as abinder, and a further 40 minutes of kneading was performed at a rotationspeed of 40 rpm. Deionized water was added to adjust the viscosity to3,000±500 mPa·s (measured by a B-type viscometer at 25° C. and 60 rpm)and thereby produce a slurry composition for a negative electrode mixedmaterial layer.

A comma coater was then used to apply the obtained slurry compositionfor a negative electrode mixed material layer onto the surface of copperfoil (thickness: 15 μm) serving as a current collector so as to have acoating weight of 11±0.5 mg/cm². The copper foil with the slurrycomposition for a negative electrode mixed material layer appliedthereon was subsequently conveyed inside an 80° C. oven for 2 minutesand inside a 100° C. oven for 2 minutes at a speed of 500 mm/min so asto dry the slurry composition on the copper foil and thereby obtain anegative electrode web including a negative electrode mixed materiallayer formed on the current collector.

The negative electrode mixed material layer-side of the producednegative electrode web was subsequently roll pressed in an environmenthaving a temperature of 25±3° C. and with a line pressure of 11 t (tons)to obtain a negative electrode having a negative electrode mixedmaterial layer density of 1.60 g/cm³. Thereafter, the negative electrodewas left for 1 week in an environment having a temperature of 25±3° C.and a relative humidity of 50±5%.

<Preparation of Separator>

A separator made from polypropylene (product name: Celgard® 2500(Celgard is a registered trademark in Japan, other countries, or both))was prepared as a separator.

<Production of Lithium Ion Secondary Battery>

The positive electrode that had been left as described above was cut outas 49 cm×4.5 cm and was placed on a horizontal stage such that thesurface at the positive electrode mixed material layer-side was on top.The separator described above was cut out as 120 cm×5.0 cm and wasarranged on the positive electrode such that the positive electrode waspositioned at a longitudinal direction left-hand side of the separator.The negative electrode that had been left as described above was cut outas 50 cm×4.7 cm and was arranged on the separator such that the surfaceat the negative electrode mixed material layer-side faced toward theseparator and such that the negative electrode was positioned at alongitudinal direction right-hand side of the separator. A windingmachine was used to wind the resultant product with the middle of theseparator in the longitudinal direction at the center to obtain a roll.The roll was pressed into a flattened form at 60° C. and 0.5 MPa and wasthen enclosed in an aluminum packing case serving as a battery case. Thealuminum packing case was then filled with a LiPF₆ solution of 1.0 M inconcentration (solvent: mixed solvent of ethylene carbonate/diethylcarbonate=3/7 (volume ratio); additive: containing 2 volume% (solventratio) of vinylene carbonate) as an electrolyte solution. The aluminumpacking was then closed by heat sealing at a temperature of 150° C. totightly seal an opening of the aluminum packing, and thereby produce alithium ion secondary battery. The lithium ion secondary battery wasused to evaluate rate characteristics, high-temperature storagecharacteristics, and safety. The results are shown in Table 1.

Examples 2 to 4

A positive electrode, a negative electrode, and a separator wereprepared and a lithium ion secondary battery was produced in the sameway as in Example 1 with the exception that the polymer B, the polymerC, or the polymer D was used instead of the polymer A as a binder inproduction of the positive electrode. The evaluations were alsoperformed in the same way as in Example 1. The results are shown inTable 1.

Examples 5 and 6

A positive electrode, a negative electrode, and a separator wereprepared and a lithium ion secondary battery was produced in the sameway as in Example 1 with the exception that melamine cyanurate (foamingtemperature: 300° C.; average particle diameter: 2 μm; nitrogen content:49 weight %) or trihydrazine triazine (foaming temperature: 270° C.;average particle diameter: 5 μm; nitrogen content: 74 weight %) was usedinstead of azodicarbonamide as a foaming agent in production of thepositive electrode. The evaluations were also performed in the same wayas in Example 1. The results are shown in Table 1.

Example 7

A positive electrode, a negative electrode, and a separator wereprepared and a lithium ion secondary battery was produced in the sameway as in Example 1 with the exception that a foaming agent was not usedin production of the positive electrode and a slurry composition for anegative electrode mixed material layer obtained by adding 0.5 parts ofazodicarbonamide (foaming temperature: 200° C.) as a foaming agent atthe same time as the polymer E as a binder was used in production of thenegative electrode. The evaluations were also performed in the same wayas in Example 1 with the exception that the content of organonitrogenfoaming agent in the negative electrode mixed material layer wasevaluated instead of that in the positive electrode mixed materiallayer, and peel strength and volume resistivity of the negativeelectrode were evaluated instead of those of the positive electrode. Theresults are shown in Table 1.

Examples 8 and 9

A positive electrode, a negative electrode, and a separator wereprepared and a lithium ion secondary battery was produced in the sameway as in Example 7 with the exception that the polymer F or the polymerG was used instead of the polymer E as a binder in production of thenegative electrode. The evaluations were also performed in the same wayas in Example 7. The results are shown in Table 1.

Examples 10 and 11

A positive electrode, a negative electrode, and a separator wereprepared and a lithium ion secondary battery was produced in the sameway as in Example 7 with the exception that the amount ofazodicarbonamide used as a foaming agent in production of the negativeelectrode was changed to 2 parts or 5 parts. The evaluations were alsoperformed in the same way as in Example 7. The results are shown inTable 1.

Example 12

A positive electrode, a negative electrode, and a separator wereprepared and a lithium ion secondary battery was produced in the sameway as in Example 7 with the exception that sodium hydrogen carbonate(foaming temperature: 150° C.; average particle diameter: 10 μm;released amount of carbon dioxide gas: 130 mL/g) was used instead ofazodicarbonamide as a foaming agent in production of the negativeelectrode. The content of inorganic foaming agent in the negativeelectrode mixed material layer was evaluated and other evaluations wereperformed in the same way as in Example 7. The results are shown inTable 1.

Comparative Example 1

A positive electrode, a negative electrode, and a separator wereprepared and a lithium ion secondary battery was produced in the sameway as in Example 1 with the exception that a foaming agent was not usedin production of the positive electrode. The evaluations were alsoperformed in the same way as in Example 1. The results are shown inTable 1.

Comparative Example 2

A positive electrode, a negative electrode, and a separator wereprepared and a lithium ion secondary battery was produced in the sameway as in Example 1 with the exception that polyvinylidene fluoride(glass-transition temperature: −40° C.) was used instead of the polymerA as a binder in production of the positive electrode. The evaluationswere also performed in the same way as in Example 1. The results areshown in Table 1.

In Table 1, shown below:

“NCA” indicates lithium-containing complex oxide of Ni-Co-Al;

“PVDF” indicates polyvinylidene fluoride;

“ST” indicates styrene unit;

“BD” indicates 1,3-butadiene unit;

“Hydrogenated BD” indicates hydrogenated 1,3-butadiene unit;

“AN” indicates acrylonitrile unit;

“MAA” indicates methacrylic acid unit;

“IA” indicates itaconic acid unit;

“2-HEA” indicates 2-hydroxyethyl acrylate unit;

“AAm” indicates acrylamide unit;

“ADCA” indicates azodicarbonamide;

“MC” indicates melamine cyanurate;

“THT” indicates trihydrazine triazine; and

“NaHCO₃” indicates sodium hydrogen carbonate.

TABLE 1 Example 1 Example 2 Example 3 Example 4 Example 5 Example 6Electrode for Type (evaluated electrode) Positive Positive PositivePositive Positive Positive electrochemical electrode electrode electrodeelectrode electrode electrode device Electrode Electrode Type NCA NCANCA NCA NCA NCA mixed active material material layer Binder Type PolymerA Polymer B Polymer C Polymer D Polymer A Polymer A Repeating ST [mass%] — — — — — — units BD [mass %] 1.5 1.8 1.3 — 1.5 1.5 Hydrogenated BD38.5 48.2 58.7 — 38.5 38.5 [mass %] AN [mass %] 60 50 40 62 60 60 MAA[mass %] — — — 2 — — IA [mass %] — — — — — — 2-HEA [mass %] — — — 35 — —AAm [mass %] — — — 1 — — Diene monomer unit + nitrile 61.5 51.8 41.3 6261.5 61.5 group-containing monomer unit [mass %] Glass-transitiontemperature 7.8 −12 −29 66 7.8 7.8 [° C.] Content per 100 parts of 2 2 22 2 2 electrode active material [parts by mass] Foaming Type ADCA ADCAADCA ADCA MC THT agent Foaming temperature [° C.] 200 200 200 200 300270 Content per 100 parts of 0.8 0.8 0.8 0.8 0.8 0.8 electrode activematerial [parts by mass] Volume resistivity R_(A) (25° C.) [Ω · cm] 8090 85 100 120 160 Volume resistivity R_(B) (350° C.)/Volume resistivityR_(A) 300 330 380 30 120 50 (25° C.) [—] Evaluation Peel strength A B CC A A Rate characteristics A A A A B C High-temperature storagecharacteristics A B C A A A Safety A A A C B C Example 7 Example 8Example 9 Example 10 Example 11 Electrode for Type (evaluated electrode)Negative Negative Negative Negative Negative electrochemical electrodeelectrode electrode electrode electrode device Electrode Electrode TypeGraphite Graphite Graphite Graphite Graphite mixed active materialmaterial layer Binder Type Polymer E Polymer F Polymer G Polymer EPolymer E Repeating ST [mass %] 64 32 32 64 64 units BD [mass %] 32 3754 32 32 Hydrogenated BD — — — — — [mass %] AN [mass %] — — 10 — — MAA[mass %] — 30 — — — IA [mass %] 3 — 3 3 3 2-HEA [mass %] 1 1 1 1 1 AAm[mass %] — — — — — Diene monomer unit + nitrile 32 37 64 32 32group-containing monomer unit [mass %] Glass-transition temperature 1737 −23 17 17 [° C.] Content per 100 parts of 1 1 1 1 1 electrode activematerial [parts by mass] Foaming Type ADCA ADCA ADCA ADCA ADCA agentFoaming temperature [° C.] 200 200 200 200 200 Content per 100 parts of0.5 0.5 0.5 2 5 electrode active material [parts by mass] Volumeresistivity R_(A) (25° C.) [Ω · cm] 1 1.2 0.8 3.5 5 Volume resistivityR_(B) (350° C.)/Volume resistivity R_(A) 270 180 350 300 400 (25° C.)[—] Evaluation Peel strength A B B B C Rate characteristics A A A B CHigh-temperature storage characteristics A A B A A Safety A B A A AComparative Comparative Example 12 Example 1 Example 2 Electrode forType (evaluated electrode) Negative Positive Positive electrochemicalelectrode electrode electrode device Electrode Electrode Type GraphiteNCA NCA mixed active material material layer Binder Type Polymer EPolymer A PVDF Repeating ST [mass %] 64 — units BD [mass %] 32 1.5Hydrogenated BD — 38.5 [mass %] AN [mass %] — 60 MAA [mass %] — — IA[mass %] 3 — 2-HEA [mass %] 1 — AAm [mass %] — — Diene monomer unit +nitrile 32 61.5 0 group-containing monomer unit [mass %]Glass-transition temperature 17 7.8 −40 [° C.] Content per 100 parts of1 2 2 electrode active material [parts by mass] Foaming Type NaHCO₃ —ADCA agent Foaming temperature [° C.] 150 — 200 Content per 100 parts of0.5 — 0.8 electrode active material [parts by mass] Volume resistivityR_(A) (25° C.) [Ω · cm] 0.9 110 175 Volume resistivity R_(B) (350°C.)/Volume resistivity R_(A) 13 8 15 (25° C.) [—] Evaluation Peelstrength A D E Rate characteristics C B C High-temperature storagecharacteristics B B D Safety C E E

It can be seen from Table 1 that the electrodes of Examples 1 to 12,which each include an electrode mixed material layer containing anelectrode active material, a specific binder, and a foaming agent on acurrent collector, and for each of which the volume resistivity R_(A)(25° C.) of an electrode laminate is within a range of 0.1 Ω·cm to 200Ω·cm and the value of volume resistivity R_(B) (350° C.)/volumeresistivity R_(A) (25° C.) is 10 or more, have excellent peel strengthand can provide a lithium ion secondary battery with a high level ofsafety. It can also be seen that the electrodes of Examples 1 to 12 cancause a lithium ion secondary battery to display excellent ratecharacteristics and high-temperature storage characteristics.

On the other hand, it can be seen that the positive electrode ofComparative Example 1, which does not contain a foaming agent in thepositive electrode mixed material layer and has a value of less than 10for volume resistivity R_(B) (350° C.)/volume resistivity R_(A) (25° C.)of an electrode laminate, has poor peel strength and cannot provide alithium ion secondary battery with a high level of safety.

Moreover, it can be seen that the positive electrode of ComparativeExample 2, which includes a positive electrode mixed material layerformed using polyvinylidene fluoride instead of the specific binder, haspoor peel strength, cannot provide a lithium ion secondary battery witha high level of safety, and cannot cause a lithium ion secondary batteryto display good enough high-temperature storage characteristics.

INDUSTRIAL APPLICABILITY

According to the present disclosure, it is possible to provide anelectrode for an electrochemical device that has excellent peel strengthand can ensure a high level of safety of an electrochemical device, andalso to provide a method of producing this electrode for anelectrochemical device.

Moreover, according to the present disclosure, it is possible to providean electrochemical device in which a high level of safety is ensured.

The invention claimed is:
 1. A positive electrode for an electrochemicaldevice comprising a current collector and a positive electrode mixedmaterial layer on the current collector, wherein the positive electrodemixed material layer contains a positive electrode active material, abinder, and a foaming agent, the binder is a polymer including a nitrilegroup-containing monomer unit, and in which a proportion constituted bya diene monomer unit and a proportion constituted by the nitrilegroup-containing monomer unit are, in total, not less than 55 mass % andnot more than 80 mass %, and in which a proportion constituted by thenitrile group-containing monomer unit is 55 mass % or more, when theamount of all repeating units included in the polymer is taken to be 100mass %, and volume resistivity R_(A) of a laminate of the positiveelectrode mixed material layer and the current collector at 25° C. isnot less than 0.1 Ω·cm and not more than 200 Ω·cm, and a ratio of volumeresistivity R_(B) of the laminate at 350° C. relative to the volumeresistivity R_(A) of the laminate at 25° C. is 10 or more.
 2. Thepositive electrode for an electrochemical device according to claim 1,wherein the foaming agent has a foaming temperature of not lower than100° C. and not higher than 350° C.
 3. The positive electrode for anelectrochemical device according to claim 1, wherein the positiveelectrode mixed material layer contains not less than 0.01 parts by massand not more than 10 parts by mass of the foaming agent per 100 parts bymass of the positive electrode active material.
 4. The positiveelectrode for an electrochemical device according to claim 1, whereinthe foaming agent is an organonitrogen foaming agent.
 5. The positiveelectrode for an electrochemical device according to claim 1, whereinthe polymer has a glass-transition temperature of not lower than −30° C.and not higher than 100° C.
 6. The positive electrode for anelectrochemical device according to claim 1, wherein the volumeresistivity R_(A) of the laminate at 25° C. is not less than 10 Ω·cm andnot more than 180 Ω·cm.
 7. An electrochemical device comprising thepositive electrode for an electrochemical device according to claim 1.8. A method of producing the positive electrode for an electrochemicaldevice according to claim 1, comprising: applying a slurry compositionfor a positive electrode mixed material layer containing the positiveelectrode active material, the binder, the foaming agent, and a solventonto the current collector; and drying the slurry composition for apositive electrode mixed material layer applied on the current collectorat a temperature of not lower than 50° C. and not higher than 130° C. toform a positive electrode mixed material layer.